Many wireless communication systems use microwave integrated circuits (MIC) and multichip microwave modules to generate and process transmitted and received communication signals. Wireless communication signals generally occupy the RF and microwave frequencies of the spectrum, although developments in wireless communications include the implementation of systems and signals operating in the millimeter wavelength frequency range. As wireless communication becomes more prevalent, it is desirable to reduce the physical size of the communication devices so they can be installed into daily operations unobtrusively. Accordingly, there is industry pressure to miniaturize microwave integrated circuits and microwave multichip modules that make up constituent parts of wireless communication devices and systems. It is also desirable to integrate functionality of the MICs and microwave multichip modules and supporting circuitry into smaller packages. A wireless communication signal generated on an MIC requires an appropriate launch into the air for practical use. Conventionally, an electronic signal is carried via a coaxial connection from the transmitter/receiver circuit to an external antenna in order to achieve adequate signal integrity in the process of the signal launch. In the interest of further system integration and miniaturization, however, it is desirable to integrate an MIC and microwave multichip module with a waveguide launch, so a signal may be launched and received directly to and from the MIC and microwave multichip module. There is a need, therefore, for a practical method for conversion of an RF, microwave, or millimeter wave signal from a signal on an MIC to a radiated wave suitable for launch as a communications signal. There is a need, therefore, for a practical conversion from a signal travelling in a conductive metal strip or wire directly to a waveguide that may be part of the microwave multichip module and then air.
A known conversion is an E-field or E-plane probe method in which the center conductor of a coaxial cable or a coplanar line is positioned in the interior of a waveguide cavity. One end of the waveguide cavity is shorted. Signals in the probe produce an electric field and excite fields in the waveguide that are directly related to the signal. Accordingly, a certain amount of direct coupling can be achieved. Disadvantageously, the E-field probe method of transformation is bandwidth limited and requires complex assembly that is relatively intolerant to manufacturing tolerances due to the importance of the position of the probe in the cavity to achieve maximum coupling.
Another known conversion is disclosed in U.S. Pat. Nos. 2,825,876, 3,969,691, and 4,754,239 and is termed a xe2x80x9cridge transitionxe2x80x9d. The ridge transition comprises a signal line supported by a dielectric substrate and positioned parallel to a ground plane on an opposite side of the dielectric in a microstrip configuration. An end of the microstrip abuts a waveguide cavity and a conducting ridge is positioned at the end of the microstrip and within the waveguide cavity. Although this method produces the desired conversion from microstrip to waveguide, the fabrication, positioning, alignment, and tolerancing of the conducting ridge renders the manufacture and assembly of the part complex and impractical for volume manufacturing.
Another known conversion is disclosed in MTT-S 1998 International Microwave Symposium Digest paper entitled xe2x80x9cA Novel Coplanar Transmission Line to Rectangular Waveguidexe2x80x9d by Simon, Werthen, and Wolff. The transformer comprises a microstrip line supported by a dielectric substrate. On an opposite side of the substrate, there are two printed conductive patches positioned in a waveguide cavity. The signal travelling in the microstrip induces a current in the patches that is coupled to the other patch. By proper choice of the patch separation constructive interference of the RF signal is achieved in the waveguide. Disadvantageously, the structure disclosed has significant insertion loss at higher frequencies and a relatively narrow bandwidth of operation. Although the disclosed design has a simpler structure than the other prior art transformers, it is relatively sensitive to manufacturing tolerances and operating environment. In addition, the transition also exhibits higher radiation and thereby reduced isolation and increased loss.
Another challenge associated with the launch of a signal present on a MIC to a wireless communication signal is that there is a significant impedance mismatch between a conventional 50 ohm transmission line and a much higher 377 ohms impedance in free space. Impedance mismatch results in a reduction of system bandwidth, which compromises the capability of the system to support high speed transmissions. Conventionally, a series of impedance steps is designed into a system to gradually transition a low impedance transmission medium to the final high impedance transmission medium. The gentler the taper, the better the match, and the greater the system bandwidth. Disadvantageously, the gentler the taper, the greater the amount of physical space is needed to accommodate the taper and the larger the overall system. There is a need, therefore, for a method of tapering the impedance mismatch from a transmission line to a radiating waveguide, which occupies a minimum amount of space while preserving adequate bandwidth.
There remains a need for a broadband manufacturable microstrip to waveguide transition for high frequency MICs and microwave multichip modules.
It is an object of an embodiment according to the teachings of the present invention to provide a transition from a planar transmission line signal to a waveguide signal and then to a radiated signal in air that is simply manufactured and relatively insensitive to manufacturing tolerances.
A transition from a planar transmission line to a waveguide comprises a planar transmission line disposed on a substrate and a mode transformer to convert a transverse electric or quasi-transverse electric mode signal carried by the transmission line to a waveguide mode signal. A first impedance matching element comprises a combination of a first extension of the substrate and a dielectric portion having a first depth. A second impedance matching element comprises a combination of a second extension of the substrate and a dielectric portion having a second depth, the second depth being greater than the first depth.
It is a feature of an embodiment according to the teachings of the present invention that a substrate on which an IC can be disposed also comprises a portion of an impedance matching element for converting a signal traveling in a planar transmission line to a signal appropriate for wireless communication.
It is a feature of an embodiment according to the teachings of the present invention that practical use of the substrate as both substrate and impedance match element provides a compact design with acceptable RF loss performance.
It is an advantage of an embodiment according to the teachings of the present invention that a vertically oriented waveguide can be realized using conventional planar manufacturing techniques.
It is an advantage of an embodiment according to the teachings of the present invention that a broadband millimeter wave waveguide transition can be realized using relatively low cost manufacturing techniques.